1. Field of the Invention
The present invention relates generally to cardiac stimulating devices. More particularly, the present invention relates to an improved protocol for a diagnostic test performed by an implantable medical device.
2. Description of the Related Art
In the normal human heart, illustrated in FIG. 1, the sinus (or sinoatrial (SA)) node, which is generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrioventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract, the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs, and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. Four one-way valves, between the atrial and ventricular chambers in the right and left sides of the heart (the tricuspid valve and the mitral valve, respectively), and at the exits of the right and left ventricles (the pulmonic and aortic valves, respectively, not shown) prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed normal sinus rhythm ("NSR") or simply sinus rhythm. This capacity to produce spontaneous cardiac impulses is called "rhythmicity", or "automaticity." Certain other cardiac tissues possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
Disruption of the natural pacemaking and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. An artificial pacemaker (or "pacer" as it is commonly labeled) is a medical device which delivers electrical pulses to an electrode that is implanted adjacent to or in the patient's heart in order to stimulate the heart's SA node so that it will contract and beat at a desired rate. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006.
Pacers today are typically designed to operate using one of three different response methodologies, namely, asynchronous (fixed rate), inhibited (stimulus generated in the absence of a specified cardiac activity), or triggered (stimulus delivered in response to a specified parameter). Broadly speaking, the inhibited pacemakers are "demand" type pacemakers, in which a pacing pulse is only generated when demanded by the heart. To determine when the heart requires pacing, demand pacemakers may sense various conditions such as heart rate, physical exertion, temperature, and the like. Moreover, pacemaker implementations range from the simple fixed rate, single chamber device that provides pacing with no sensing function, to highly complex models that provide fully automatic dual chamber pacing and sensing functions. The latter type of pacemaker is the latest in a progression toward physiologic pacing, that is, the mode of artificial pacing that most closely simulates natural pacing.
Because of the large number of options available for pacer operation, an industry convention has been established whereby specific pacer configurations are identified according to a code comprising three or four letters. A fifth coded position may be used to describe a pacemaker's ability to respond to abnormally high heart rates (referred to as tachycardia). Because most pacemakers do not provide any antitachycardia functions, the fifth coded position is not used in most commonly used pacemaker types. Thus, most common configuration codes comprise either three or four letters, as shown in Table I below. For this reason and for simplicity's sake, the fifth code position is omitted from the following table. Each code can be interpreted as follows:
TABLE I ______________________________________ Code 1 2 3 4 position Function chamber chamber response to programmability, Identified paced sensed sensing rate modulation Options 0--none 0--none 0--none 0--none Available A--atrium A--atrium T-- P--programmable V--ventricle V--ventricle triggered M--multi- D--dual D--dual I--inhibited programmable (A + V) (A + V) D--dual C-- (T + I) communicating R--rate modulating ______________________________________
For example, a DDD pacer paces both chambers (atrium or ventricle) and senses in both either chambers. Thus, a pacer in DDD mode, may pace the ventricle in response to electrical activity sensed in the atrium. A VVI pacer paces and senses in the ventricle, but its pacing can be inhibited by spontaneous electrical activation of the ventricle (i.e., the ventricle paces itself naturally). In VVIR mode, ventricular pacing is similarly inhibited upon determining that the ventricle is naturally contracting. With the VVIR mode, the pacer's pacing rate, however, in the absence of naturally occurring pacing, is modulated by the physical activity level of the patient. Pacers commonly include accelerometers to provide an indication of the patient's level of physical activity.
As illustrated in the table above, it may be desired to sense in one cardiac chamber (i.e., detect electrical activity representative of contraction of the chamber and referred to as a "sensed event") and, in response, pace (referred to as a "paced event") in the same or a different chamber. In general, most pacemakers today incorporate a sensing function to detect electrical activity at the site of one or more electrodes. The sensing circuit in the pacemaker (often referred to as the "sense" circuit) receives the electrical signals from the electrodes and determines when a physiologically significant event as occurred. Accordingly, if the heart's natural pacemaker is able to stimulate the heart to beat properly, the pacemaker's sense circuit detects the naturally occurring electrical impulses and determines that the heart does not require artificial stimulation.
Most pacemaker sense circuits incorporate an amplifier that amplifies the electrical signals received from the electrodes. Sense circuits typically also incorporate, or are coupled to, a comparator circuit that compares the magnitude of the amplified signal from an electrode to a reference signal. When the amplified signal from the electrode exceeds the amplitude of the reference signal, the pacemaker determines that a physiologically significant event has occurred. In this context, the physiologically significant events are cardiac events. It is important for a pacemaker to accurately determine when a cardiac event has occurred. This means that the pacemaker should detect a true cardiac event, but not respond to non-cardiac signals.
The sense circuit's ability to accurately determine when a cardiac significant event has occurred may be compromised because the electrodes are sensitive to various sources of electrical signals. For example, muscle tissue generates electrical signals referred to as electromyogram ("EMG") signals. These EMG signals may be picked up by the pacemaker electrodes and amplified along with the naturally occurring cardiac signals. By way of further example, electromagnetic interference (EMI) signals generated outside the patient's body may also be detected by the pacemaker's electrodes.
For a sense circuit to accurately respond to cardiac signals, the circuit's sensitivity must be set at an optimal level. That is, the sensitivity must be set high enough so that all cardiac events will be detected, but not so high that other non-cardiac related events are detected and falsely determined to have a cardiac origin. Thus, upon implantation into the body, the surgeon monitors the pacemaker and sets its sensitivity appropriately.
A physician may also monitor the pacemaker during post-operative examinations. During such an examination, the physician may determine if the sensitivity of the pacemaker's sense circuit is set correctly, or needs to be reset. Most pacemakers are capable of performing a sensitivity test that is initiated by an external programmer controlled by the physician. The programmer includes a "wand" that is positioned over the site on the chest in which the pacemaker is implanted. Using any one of a variety of wireless communication techniques, commands and data can be transmitted between the implanted pacemaker and the wand. Using the external programmer, the physician can command the pacemaker to begin a sensing test. During the test, the pacemaker's sense circuit monitors the heart's naturally occurring electrical rhythm and generates data based on the amplitude of the heart's signal relative to the internal reference signal. The programmer periodically transmits a command to the pacemaker directing the pacemaker to transmit the data to the programmer. The programmer analyses this data to compute an incremental change to the sensitivity setting and transmits the newly computed sensitivity setting back to the pacemaker. The programmer also analyzes the data provided by the pacemaker to determine if the previously transmitted sensitivity setting was optimal. Once the external programmer determines that the sensitivity of the pacemaker is at an optimal setting, the programmer terminates the sensing test. The external programmer includes sufficient data storage and processing capability to receive the sensing test data and calculate new sensitivity settings.
Unfortunately, the signals that the external programmer repeatedly transmits during the sensing test may interfere with the accuracy of the test itself. The implanted pacemaker's electrodes are sensitive to any electrical signal in the vicinity of the electrode, and thus the electrodes may pick up the command and sensitivity signals that are being transmitted to the pacemaker by the programmer. The programmer's signals combine with the heart's naturally occurring cardiac signals and are provided to the sense circuit via the electrodes. All signals impinging on the electrodes, including any control and data signals generated by the external programmer are processed by the pacemaker (and thus the programmer) during the sensing test. The programmer will compute new sensitivity values based on data that has been distorted by the programmer itself. Thus, if programmer-initiated signals are processed by the pacemaker during the sensing test, the accuracy of the test itself is detrimentally effected. In other words, the very device that is employed to initiate and control the sensing test (i.e., the external programmer) introduces errors into the test.
For these reasons, an implantable medical device is needed that can perform a sensing test without introducing errors into the test from control signals transmitted from the programmer to the pacemaker during the test. Despite the advantages such a device would provide, no such device is known to exist today.